Germline mutations in the BRCA1 and BRCA2 genes account for 70-80% of hereditary breast and ovarian cancers. Both are thought to be tumor suppressor genes as the wild-type alleles are lost in tumors of heterozygous carriers. Extensive research efforts from all disciplines contributed a plethora of data suggesting a role for both BRCA1 and BRCA2 in embryonic proliferation, homologous recombination, DNA repair, cell cycle checkpoint control, and transcriptional regulation. However, it is unclear which function(s) specifically suppress tumorigenesis. Biochemical studies have identified a number of BRCA1-interacting proteins that may function in Brcalmediated tumorigenesis. Two of these proteins, BARD1 and BACH1, were found to be mutated in human breast cancer, suggesting that they may also serve as tumor suppressors in vivo. Notably, the heterodimeric BRCA1/BARD1 complex was recently shown to possess E3-ubiquitin ligase activity, while the putative DNA helicase BACH1 appears to be critical for BRCA1 function in maintenance of genomic integrity. The tumor suppressor protein p53 is mutated in a large number of BRCA1- and BRCA2-associated tumors consistent with the hypothesis that inactivation of a cell cycle checkpoint is a necessary step and may precede BRCA loss during tumorigenesis. To test these hypotheses, three Specific Aims will be pursued. First, to inactivate the Bardl gene specifically in mammary epithelial cells by conditional mutagenesis, in order to bypass the early embryonic lethality associated with global ablation of its function and study the possible development of mammary tumors. Second, to generate Bach1-deficient mice to investigate the role of Bach1 in mammary tumor suppression. Third, to use Brca1, Brca2, Bard1, Bach1 and p53 conditional mutant mice in a breeding program to assess combinatorial effects of mutations in vivo on tumor initiation and progression. Following histopathological grading of primary mammary tumors, we will use oligonucleotidebased microarray technology (Affimetrix) to compare and define changes in the gene expression profile and to uncover the signaling pathways relevant to BRCA1/2 mediated tumor suppression.